Literally a "river of fire" (Aen. 6.550-1), Phlegethon is the name
Dante gives to the river of hot blood that serves as the first ring of
circle 7: spillers of blood themselves, violent offenders against others
are submerged in the river to a level corresponding to their guilt. Dante
does not identify the river--described in detail in Inferno 12.46-54
and 12.100-39--until the travelers have crossed it (Dante on the back of
Nessus) and passed through the forest of the suicides. Now they approach
a red stream flowing out from the inner circumference of the forest across
the plain of sand (Inf. 14.76-84). After Virgil explains the common
source of all the rivers in hell, Dante still fails to realize--without
further explanation--that the red stream in fact connects to the broader
river of blood that he previously crossed, now identified as the Phlegethon
(Inf. 14.121-35).